1. Field of the Invention
This invention relates to a process for producing carbon nanotubes.
During the last ten years or so, carbon nanotubes have generated a substantial amount of interest in the scientific community. The high chemical stability, mechanical strength and electrical conductivity of single walled (SWCNT) and multiwalled carbon nanotubes (MWCNTs) make the material attractive for industrial applications such as microelectronics, flat panel display, the strengthening of composite materials, hydrogen storage and electrodes for batteries and fuel cells.
2. Discussion of the Prior Art
At present, carbon nanotubes are produced by laser ablation, ablation using an electric arc and chemical vapor deposition. Examples of these method are described in American Scientist 85 (1997) 324-337; Journal of Molecular Catalysts A: Chemical 116 (1997) 397-403; Nature 354 (1991) 56; Nature 358 (1992) 220; Nature 363 (1993) 603; Chemical Phys. Lett. 289 (1998) 602; Appl. Phys. Lett 75 (1999) 367; Carbon 35 (1997) 1495; Published International Patent Applications WO 99/06618, Feb. 11, 1999 and WO 99/25652, May 27, 1999 and U.S. Pat. No. 5,780,101, issued Jul. 14, 1998 and U.S. Pat. No. 5,965,267, issued Oct. 12, 1999.
The first two methods, i.e. laser and electric arc ablation are based on the same principle, namely the evaporation of a graphite rod enriched with a metallic catalyst, and condensation of the vapor thus produced in nanotube form. Such methods, which permit the fabrication of bulk multilayer nanotubes, require large amounts of energy for the evaporation process as opposed to the chemical vapor deposition (CVD) method. In fact, in order to achieve atomic vaporization of one mole of carbon, 717 kJ is required, while with the CVD method 227 kJ is generated by the decomposition of one mole of acetylene molecules. With the CVD method, it is possible to control the area where nanotubes are deposited on a surface [J. Am. Chem. Soc. 121(1999) 10832 and Science 282 (1998) 1105]. The problems associated with the CVD method include low yield and the generation of large quantities of amorphous carbon during pyrolysis of the carbon containing gas.
Applicants' and others previously proposed a process involving the deposition of a metal catalyst such as iron or nickel on carbon paper, and passing a feedstock gas containing a source of carbon over the substrate while applying an electrical current thereto to heat the substrate sufficiently to generate a reaction between the catalyst and the feedstock gas. [see “Growth of Carbon Nanotubes on Ohmically Heated Carbon Paper”, Chem. Phys. Lett, Vol 342, No 5-6, p503-509 (2001)]
It has been found that carbon nanotubes produced using the previously proposed process contain a rather large amount of defects, and undesirable amorphous carbon.